The present invention relates to a coating method for coating metal plates and in particular, to a coating method for coating metal plates exhibiting excellent electrodeposition coating and workability, which comprises coating a sheet steel, a steel article, or a plated article thereof; an aluminum article, a zinc article, or an alloy article thereof; a cold rolled bright sheet steel or an alloy-plated cold rolled bright sheet steel; or the like with an organic coating composition containing molybdenum disulfide alone or molybdenum disulfide in combination with electroconductive fine particles, drying the thus coated sheet steel or article, and then subjecting it to forming processing.
Hitherto, in coating automobile bodies, a method in which a cold rolled dull sheet steel is successively subjected to surface preparation and electrodeposition coating and then finished by applying an intermediate coat and a top coat has been commonly employed. Recently, improvements in smoothness and image clearness of coating films are being keenly demanded. For this purpose, not only paints themselves but smoothness of a substrate have come into question. However, since a cold rolled bright sheet steel which exhibits the highest smoothness among steels does not have holding power for lubricants, it involves problems from the standpoint of processing steps because it likely causes inconveniences such as a phenomenon in which a material to be processed adheres to the surface of a mold to damage the mold, and a phenomenon in which a material to be processed adheres to the surface of a mold during the forming processing. Accordingly, although the cold rolled bright sheet steel is known to be the best steel in order to improve the image clearness, it has not yet been used. Recently, a laser dull sheet steel comprising a sheet steel having provided thereon regular markings has turned up. Through this laser dull sheet steel is admitted to have an effect for improving the sharpness as compared with the cold rolled dull sheet steel, it is still not superior to the cold rolled bright sheet steel.
Furthermore, in organic coating film-applied composite plated sheet steels used in automobile bodies, if a substrate sheet steel is a cold rolled dull sheet steel, it is known that there are involved the same problems in smoothness and image clearness. Still further, in the organic coating film-applied composite plated sheet steels, coating films containing a large quantity of zinc dust such as zincrometal generate problems such as peeling and powdering during the forming processing. Moreover, even in composite coated sheet steels having a 1.mu.-thick silica-containing organic coating film which has been developed thereafter, since the coating film is electrically insulative, in order to obtain a good property of electrodeposition coating in the electrodeposition coating to be subsequently carried out, the film thickness must be controlled within 1.+-.0.3 microns, whereby a large number of management steps are required for the production so that even a slight dispersion variability of the film thickness results in deterioration of the property of electrodeposition coating. In any of these cases, demands the present status an immediate improvement.
There have been made investigations based on an assumption that in sheet steels for automobile bodies having these defects,if a phenomenon in which a material to be processed adheres to the surface of a mold to damage the mold, and a phenomenon in which a material to be processed adheres to the surface of a mold during the forming processing, could be solved by coating, the problems in smoothness and image clearness of the finishing of a top coat could be solved. As a result, it has become clear that in the case that an organic lubricant is added merely for the purpose of imparting workability, though the workability is improved, there is involved a problem in the property of electrodeposition coating, whereas in the case that a coating composition to which electrical conductivity has been imparted is applied on a sheet steel, though the property of electrodeposition coating is improved, the workability is not improved at all.